The proposed research is aimed primarily at the study of a) the mechanisms responsible for the formation of highly localized acetylcholine receptor clusters known to occur in developing muscle cells in vitro, b) the topographic organization and dynamic state of some of the molecular constituents of these clusters, c) the effect of maturation and innervation on this organization and d) the role of membrane fluidity in all of these phenomena. The elucidation of these problems will be approached by means of a novel biophysical strategy based on fluorescence energy tranfer phenomena which entails making nanosecond fluorescence relaxation measurements through the microscope from single living cells labelled with fluorescent probes.